The official press release came in this morning for Grand Theft Auto V, and it confirms that the game will take place in "Los Santos" and it's surrounding areas. Los Santos is a fictionalized version of southern California. Rockstar says the game is a "bold new direction" for the series. Here's a snippet from the press release.
"Developed by series creator Rockstar North, Grand Theft Auto V heads to the city of Los Santos and surrounding hills, countryside and beaches in the largest and most ambitious game Rockstar has yet created.
"Grand Theft Auto V is another radical reinvention of the Grand Theft Auto universe," said Sam Houser, Founder of Rockstar Games. "We are incredibly excited to share our new vision with our fans."
A bold new direction in open-world freedom, storytelling, mission-based gameplay and online multiplayer, Grand Theft Auto V focuses on the pursuit of the almighty dollar in a re-imagined, present-day Southern California."
No platforms or release window have been mentioned yet by Rockstar.